即使密钥存在也无法删除:错误代码:1091

我有一张像这样的桌子:

CREATE TABLE `articles` 
             ( 
                          `article_id` INT(10) NOT NULL auto_increment, 
                          `pubsrc_id`  INT(10) NOT NULL, 
                          `pub_id`     VARCHAR(16) NOT NULL
                          PRIMARY KEY (`article_id`), 
                          UNIQUE KEY `pubsrc_id` (`pubsrc_id`,`pub_id`), 
                          CONSTRAINT `pub_articles_ibfk_1` FOREIGN KEY (`pubsrc_id`) REFERENCES `pub_sources` (`pubsrc_id`) ON
             DELETE RESTRICT 
             ON 
             UPDATE RESTRICT, 
                    CONSTRAINT `pub_articles_ibfk_2` FOREIGN KEY (`pubsrc_id`) REFERENCES `pub_sources` (`pubsrc_id`)
             ) 

上一个表有另一个表的键:

         CREATE TABLE `pub_sources` 
         ( 
                      `pubsrc_id`              INT(11) NOT NULL, 
                      `pubsrc_name`            VARCHAR(32) NOT NULL
                      PRIMARY KEY (`pubsrc_id`) 
         ) 

我正试着用命令放下前导键:

SET SESSION SQL_SAFE_UPDATES = 0;
SET FOREIGN_KEY_CHECKS = 0;
ALTER TABLE `articles` DROP FOREIGN KEY `pubsrc_id`;

但得到以下错误:

Error Code: 1091. Can't DROP 'pubsrc_id'; check that column/key exists